1. Field of The Invention
This invention relates generally to cinematography; and more particularly to accessories for professional thirty-five millimeter motion-picture cameras.
The invention is an accessory for use with the cameras available commercially under the names of Arriflex.RTM. (models IIc, III and IIIc), Mitchel.RTM. (model Mark VI) and BNCR.RTM.. It permits a user to hold the viewfinder at a constant height while tilting the camera vertically, and also permits simultaneous monitoring or recording by a video camera.
2. Prior Art
Tilt viewfinders are known, as are video taps--but when one attempts to combine the two, difficulties arise. My invention resolves these problems. A review of currently available viewfinder accessories and video-camera attachment accessories will serve to delineate the problems resolved by my invention.
It is known to provide a viewfinder that can be held at constant height while the camera tilts. Arriflex offers such a finder, but it is not compatible with video monitoring.
More specifically, the Arriflex finder contains a custom prism that performs three functions (for reasons that will be discussed below). First, it deflects light from the motion-picture camera ground glass rearward (relative to the camera) along a path parallel to the side of the camera. Secondly, it then deflects the light again, outward at right angles into a viewfinder tube. Thirdly, it provides an inversion of the image.
Since a single, unitary prism performs all three functions, there is no way to tap part of the beam out of the custom prism itself to a video-camera port.
Furthermore, the forward portion of the custom prism is immediately adjacent to the ground glass; and the finder tube, similarly, is immediately adjacent to the rearward end of the prism. Hence there is no practical point in the nonpivoting part of the optical train to interpose a beam splitter, to tap part of the beam to a video port--at least not without making the unit objectionably much wider.
It might be possible to place such a splitter in the pivoting finder tube, but that would be most awkward and unsatisfactory as the user would have to manually support the video camera weight, suspended on the pivoting tube.
One might suppose that it would be possible simply to dispose of the custom Arriflex prism and place a beam-splitting prism adjacent to the ground glass--to divert part of the light upward or rearward to a video port, while passing the remainder straight out from the side of the motion-picture camera into the pivoting finder tube.
Such a solution while perhaps seductively simple is unacceptable because of the proximity of the ground glass to the front of the motion-picture camera, in combination with the necessary bulkiness of the finder-tube pivot point. In such a device the front end of the finder tube would be extremely close to the plane of the front of the camera, and might even extend beyond that plane.
In any of such positions the finder tube would interfere with installation, adjustment, operation, and/or use of many of the large lenses modernly employed in professional cinematography. Thus the first two functions of the Arriflex custom prism mentioned above are important to practical use of a tilt viewfinder, and cannot be readily eliminated in the interest of adding a video tap.
Moreover, the image-inversion provided by the Arriflex custom prism is also important. As has been mentioned, the image on the ground glass of the motion-picture camera is inverted. This inversion is corrected by a reimaging or "relay" lens in the finder tube, which lens itself provides an inversion. For purposes of simplicity in this document, I will describe provision of an inversion of an already-inverted image as "reerecting" the inverted image. Thus the relay lens reerects the ground-glass image.
Unfortunately, however, there is another image inversion in the system. It arises as follows.
A ninety-degree/forty-five-degree prism is positioned in the front end of the finder tube. This prism has the function of receiving outward-directed light rays from the Arriflex custom prism and deflecting them along the interior of the finder tube--which is, very generally, parallel to the side of the motion-picture camera. This ninety-degree/forty-five-degree prism consequently must rotate with the pivoting finder tube.
When the finder tube pivots, relative to the camera, away from a mutually level condition (that is to say, if the finder tube is not pointing straight back along the side of the camera), the ninety-degree/forty-five-degree prism in effect introduces a "twisting" of the image. In other words, the image within the viewfinder tube rotates about the optical path in proportion to the pivot angle of the tube.
To correct for this twisting of the image, a Pechan prism is mechanically rotated about the optical path within the tube, between the relay lens and the ocular. This prism, when rotated by the correct amount, counterrotates the image just enough to maintain it vertical--but not right-side-up! That is to say, this prism also introduces an inversion.
Thus the Arriflex custom prism mentioned earlier, in addition to moving the image point rearward from the ground glass to clear the front of the camera, also reerects the image to compensate for inversion at the Pechan prism. For both these reasons the Arriflex custom prism cannot be simply discarded: any workable solution must somehow preserve these functions, while adding compatibility with a video tap.
Another area of pertinent prior art is that of known "video door" accessories--so called because such units are built into panels that replace the standard "door" at the viewing side of the motion-picture camera. Video doors are available in combination with stationary viewfinders--that is to say, viewfinders that are fixed in viewing angle with respect to the body of the motion-picture camera.
The commonly encountered form of such devices consists of (1) a ninety-degree/forty-five-degree prism adjacent to the ground glass in the motion-picture camera, redirecting light from the ground glass rearward, generally parallel to the side of the motion-picture camera; (2) a cube beam splitter behind the ninety-degree/forty-five-degree prism; (3) a relay lens and ocular along the "straight through" path from the splitter; and (4) an upward-deflecting ninety-degree/forty-five-degree prism, followed by a video lens, iris and video camera, along the diverted path from the splitter.
This simple arrangement is satisfactory because the ninety-degree/forty-five-degree prism at the front of such a nonpivoting finder, next to the ground glass, does not require a bulky housing. Consequently the image need not be moved, as in the Arriflex tilt finder, rearward before passage outward into a viewfinder tube.
Now returning to discussion of the Arriflex tilt viewfinder, certain other points bear mention. It is certainly not my intention to criticize the Arriflex unit, a very fine optical instrument; nevertheless I feel that even devices of the highest quality are subject to refinement in small ways, and in the instance of the Arriflex tilt viewfinder it has been noted that the mechanical linkage that drives the Pechan prism must be of extremely high-quality construction to minimize backlash.
As will be recalled, the Pechan prism merely corrects for image twist when the viewfinder tube is pivoted relative to the camera. A small amount of backlash in the prism-drive linkage therefore only leaves the image at the ocular cocked very slightly away from vertical. It might be supposed, accordingly, that a small amount of backlash would be inconsequential.
In practice, however, small amounts of backlash in this mechanism become relatively conspicuous because if backlash is present the image-twist correction does not change smoothly and continuously with pivoting of the finder tube. The image, in effect, "jumps" or "jerks" whenever the tube pivoting direction is reversed.
Since, to the observing cinematographer, the image is always approximately vertical, these erratic dislocations are not superimposed upon a visible large rotation of the image--but rather are seen isolated, and thus appear quite conspicuous. They may also seem (perhaps quite unjustly) to represent quirky, erratic operation.
In amateur photography such matters may be a minor annoyance, but in professional cinematography such an effect is irksome to the extreme. Motion pictures are made under conditions of extraordinary demands and great tension--due to intense concentration on the technical and artistic considerations, coupled with enormous pressure of time, cost and the temperament of directors, actors and others. Within this context a jumping, jerking image in a viewfinder may become an entirely intolerable irritant.
In the Arriflex device this effect is largely avoided, but only by employing extremely fine machining tolerances, and consequently at relatively high cost. Provision of a stable image at more moderate expense would be desirable. To understand this problem it will be helpful to have an understanding of the mechanical linkage which is used in the Arriflex accessory to control the Pechan prism.
That linkage includes four ring gears, a smaller spur gear that transmits motion around a corner between the finder-tube pivot mechanism and the "downstream" parts of the tube itself, and a planetary gear. The linkage operates as follows.
One ring gear is fixed with respect to the motion-picture camera, and is coaxial with the pivoting mechanism of the viewfinder tube. The spur gear is rotatably fixed to the tube but meshed with the stationary ring gear--so that when the tube pivots, the spur must roll along the ring gear. Thus when the tube pivots, the ring gear forces the spur gear to rotate on its own axis.
The linkage also includes a metal cylinder that is mounted for rotation on its own axis, within the viewfinder tube. Ring gears are formed on both of ends of this cylinder: the ring gear on one end (the nearer) is meshed with the spur gear--so that when the tube pivots, the spur gear forces the cylinder to rotate. The ring gear on the remote end of the cylinder of course rotates as well, transmitting the pivoting motion of the tube itself to a point adjacent the Pechan prism. That remote gear may accordingly be called the motion-transmitting ring gear.
The Pechan prism is carried within a barrel which is rotatably fixed within the tube, for rotation about the optical path within the tube. A planetary gear is mounted to the periphery of this barrel for rotation with the barrel and also for rotation about the axis of the planetary gear itself. The gear axis is fixed relative to the barrel.
The planetary gear is in mesh with the ring gear on the remote end of the rotating cylinder, i.e., the motion-transmitting gear, and also in mesh with yet another ring gear--one that is fixed within the viewfinder tube. Consequently, when the tube pivots, the transmitting gear forces the axis of the planetary element to roll along the fixed gear and thus rotate about the optical path.
When the planetary moves about the optical path in this way, its axis is moving, and with it the Pechan-prism barrel mount to which the planetary axis is fixed. Hence the prism rotates about the optical path.
An important characteristic of the planetary drive--when operating against the final, fixed ring gear--is that the axis of the planetary only moves about the optical path through half the angle of rotation of the motion-transmitting gear. Consequently the barrel holding the Pechan prism likewise rotates only about the optical path through half the angle of rotation of the motion-transmitting gear. This relationship is appropriate because the Pechan prism has the property of twisting the image through an angle that is twice its own angle of rotation from the noncorrecting orientation.
From this description it may be understood that the Pechan-prism drive linkage has no fewer than four pairs of meshed gear teeth, and that the tolerances in these eight sets of teeth are additive in determining the backlash of the overall mechanism. This fact, coupled with the fact that the mechanism ends in an optical display which is virtually ideal for isolating and displaying the results of even tiny residual backlash, makes the transmission of motion to the Pechan prism an extremely fussy problem.
If economy is added to these considerations the problem becomes more than fussy. Yet resolution of this problem in an economical way is extremely desirable.
Another problem arises in the use of tilt viewfinders, even without reference to the desirability of adding video doors. This is only a problem for some practitioners, and perhaps a problem of lesser magnitude than those discussed above. This problem arises from the fact that the Arriflex tilt viewfinder accessory is aligned very close to the left side of the motion-picture camera itself.
It must be understood that on the top of the camera and extending both upward and toward the rear there is generally a very bulky film magazine. When the camera is tilted upward the film magazine moves rearward and downward. In these circumstances the viewfinder is typically being held approximately level, extending rearward from the frontal part of the left side of the camera.
Thus the film magazine moves progressively into the region immediately to the right of the ocular. If the cinematographer sights through the viewfinder with her or his right eye, the magazine clears the right side of the head by an inch or two.
To obtain this amount of clearance, the Arriflex viewfinder tube is mounted at a small angle (about ten degrees) to the side of the camera, diverging, toward the rear, from the left side of the camera. In addition the pivotal axis of the tube is at a slight angle (about six degrees) off the normal to the side of the camera--so that the finder tube swings out even further, though only slightly, as the camera tilts upward or downward.
This arrangement is regrettably quite inadequate for the cinematographer who habitually sights through the finder using the left eye. There are surprisingly many of such individuals, who must make a hard choice among three possibilities: using the right eye, squatting to bring the finder tube to a low enough point to obtain the necessary clearance, and being beheaded by the film magazine. In cinema work these choices may all be regarded as equally appealing.
Therefore, as a practical matter, providing a "left-eyed" tilt viewfinder would also be very desirable.